<!DOCTYPE html>
<html>

<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style type="text/css">
        * {
            margin: 0;
            padding: 0
        }

        ul {
            list-style: none
        }

        .wrap {
            width: 330px;
            height: 30px;
            margin: 100px auto 0;
            padding-left: 10px
        }

        .wrap li {
            background: skyblue
        }

        .wrap>ul>li {
            float: left;
            margin-right: 5px;
            position: relative;
            background: #0eb0ed
        }

        .wrap a {
            display: block;
            height: 30px;
            width: 100px;
            text-decoration: none;
            color: #000;
            line-height: 30px;
            text-align: center
        }

        .wrap li ul {
            position: absolute;
            top: 30px;
            display: none
        }
    </style>
    <script src="../jquery-1.12.2.js"></script>
    <script>
        $(() => {
            $('li:eq(0)').mouseenter(() => {
                $('li:eq(2)').parent().css('display', 'block')
            })
            $('li:eq(0)').mouseleave(() => {
                $('li:eq(2)').parent().css('display', 'none')
            })
        })
    </script>
</head>

<body>
    <div class="wrap">
        <ul>
            <li>
                <a href="javascript:void(0);">一级菜单</a>
                <ul>
                    <li><a href="javascript:void(0);">二级菜单</a></li>
                    <li><a href="javascript:void(0);">二级菜单</a></li>
                    <li><a href="javascript:void(0);">二级菜单</a></li>
                </ul>
            </li>
        </ul>
    </div>
</body>

</html>